The availability of high performance optical amplifiers such as the Erbium-Doped Fiber-Amplifier (EDFA) has facilitated continued development of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver. In a long-haul optical fiber system, the set of wavelength channels may be amplified simultaneously in an optical amplifier based repeater. The EDFA is particularly useful in this application because of its ability to amplify multiple wavelength channels with little or no crosstalk penalty.
In general, it is advantageous to operate long-haul transmission systems at a high data rate per channel. Long-haul systems may operate, for example at Synchronous Digital Hierarchy (SDH) standards up to 40 Gb/s or more. As the bit rates rise through the gigabit per second range, there is the need for an increase in the optical powers launched into the transmission fiber, e.g. to 1 mW per channel or more. As demonstrated by Cai et al. (“RZ-DPSK field trial over 13,100 km of installed non-slope-matched submarine fibers”, Journal of Lightwave Technology in Vol. 23, No. 1, January 2005 pp. 95-103), variants of the return-to-zero (RZ) modulation format are particularly useful for transmitting large amounts of data over optically amplified fiber paths.
For long distance operation at higher data rates (e.g., 40 Gb/s RZ-DPSK channels), however, there is a need to control chromatic dispersion and the dispersion slope to ensure low dispersion penalties. Dispersion shifted optical fibers have been the preferred transmission medium where there is a need to control chromatic dispersion. The combination of long distance transmission, low dispersion and high channel power may result in crosstalk, or mixing of channels through the slight nonlinearity in the transmission fiber. The transmission of many WDM channels over transoceanic distances can be limited by the nonlinear interactions between channels, which in turn is affected by the amount of dispersion. This subject was reviewed by Tkach et al. (Journal of Lightwave Technology in Vol. 13, No. 5, May 1995 pp. 841-849).
One solution to the problem of nonlinear interactions between channels is known as “dispersion mapping” where the generation of mixing products is reduced by offsetting the zero dispersion wavelength in the transmission fiber from the operating wavelengths of the transmitter. In this established technique, several amplifier sections may have dispersion shifted fiber spans with either positive or negative dispersion. The dispersion accumulates over many amplifier spans, for example, for distances of 500 to 1000 km, and the accumulated dispersion is followed by fiber with the opposite dispersion to bring the average dispersion (as averaged over the length of the cable) back to zero. One problem with this scheme is that conventional dispersion maps only compensate for the dispersion of the transmission fiber over a limited bandwidth (or a sub-set of WDM channels) while allowing the dispersion to accumulate to large values for the majority of WDM channels. To mitigate this problem, additional dispersion compensation using dispersion compensating fibers at the terminals (e.g., the transmitter and/or receiver) may be applied either before the channels are multiplexed at the transmitter, or after the channels are demuliplexed at the receiver. However, this method of dispersion compensation generally does not compensate for accumulation of dispersion slope within the optical bandwidth of a data channel.
Using these conventional dispersion management schemes, the long-haul transmission of 40 Gb/s channels suffers from dispersion-slope penalty. High speed optical data channels may require high channel power for good optical signal to noise ratio (SNR). As is well known, long optical transmission systems that suffer from optical fiber nonlinearities work better with a narrow pulse transmission format, such as RZ, CRZ, and RZ-DPSK. Unfortunately, narrow optical pulses have a wide optical spectrum. Dispersion slope causes the dispersion to change over the bandwidth of the signal having spectrally-broad pulses, which causes signal distortion and limits the ability to increase the bit rate per channel of such systems.
One method of improving 40 Gb/s operation is to use transmission fibers known as “dispersion-flattened” fibers. Unfortunately, the vast majority of existing systems that were designed to work at lower bit rates use conventional fiber that has high accumulated dispersion slope. Thus, there is a need for systems and methods to improve the performance of high-speed signals, for example, when used with conventional dispersion maps.